Novel binuclear copper(II) complexes of di-2-pyridyl ketone N(4)

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Novel binuclear copper(II) complexes of di-2-pyridyl ketone N(4)-methyl, N(4)-phenylthiosemicarbazone: Structural and

spectral investigations

Varughese Philip

^a

, V. Suni

^a

, Maliyeckal R. Prathapachandra Kurup

^a,*

, Munirathinam Nethaji

^b

aDepartment of Applied Chemistry, Cochin University of Science and Technology, Kochi 682 022, Kerala, India

bDepartment of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India

Abstract

Four binuclear complexes [Cu(dptsc)Cl]₂Æ3H₂O (1), [Cu(dptsc)Br]₂(2), [Cu(dptsc)(l-N₃)]₂(3) and [Cu(dptsc)(NO₃)]₂ÆH₂O (4) (where Hdptsc = di-2-pyridyl ketone N(4)-methyl, N(4)-phenylthiosemicarbazone) have been synthesized and physicochemically characterized. Each Cu(II) atom in the monomeric unit exists in a penta-coordinate environment. The molecular structures of [Cu(dptsc)Br]2and [Cu(dptsc)(l-N3)]2are resolved by single-crystal X-ray diﬀraction studies. Both the crystals are centrosymmetric dimers where each ligand unit coordinates through one of the pyridyl nitrogens, azomethine nitrogen and thiolate sulfur to Cu(II). A distorted square pyramidal geometry is observed around Cu(II) for both the complexes, where the N(2) nitrogen of the second ligand unit coordinates to the ﬁrst Cu(II) center in compound2and N(6) nitrogen of the azido group bridges both the Cu(II) centers in compound3. Spectral characterization corroborate the structural studies.

Keywords: Di-2-pyridyl ketone; Thiosemicarbazone; Cu(II) thiosemicarbazone complex; Crystal structure

1. Introduction

The ability of nitrogen and/or sulfur based donors to stabilize reduced and oxidized forms of copper(II) has sparked interest in their bioinorganic systems [1]. Thi- osemicarbazones and their metal complexes are nowa- days widely explored owing to their versatile biological activity and prospective use as drugs[2]. The potential anticancer, chemotherapeutic and superoxide dismu- tase-like activity of copper complexes with various thiosemicarbazones were ﬁrst reported during 1950s[3,4].

Thereafter, a great variety of biological properties rang-

ing from anticancer [5], antitumor [6], antibacterial [7], antiﬁlarial [8] and antiviral [9] activities were explored with thiosemicarbazones, while the Cu(II) derivatives of bis(thiosemicarbazone) ligands served as radiophar- maceuticals [10] and revealed hypoxic selectivity [11].

For the past decade, we have been in constant pursuit of the synthesis, structural characterization and biological studies of a variety of substituted and unsubstituted thiosemicarbazones with diﬀerent combinations of the aldehydes/ketones, with the aim to correlate their structural features with chelating ability and biological activity[12–17]. However, there are only few previous reports on di-2-pyridyl ketone thiosemicarbazones[18–22], and recently we have reported the synthesis and unique structural features of di-2-pyridyl ketoneN(4),N(4)-(bu- tane-1,4,diyl)thiosemicarbazone [23] and its Ni(II)

* Corresponding author. Tel.: +91 484 2575804; fax: +91 484 2577595.

E-mail address:[email protected](M.R. Prathapachandra Kurup).

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complexes [24]. In this paper, we report the spectral characteristics of four copper(II) complexes derived from di-2-pyridyl ketone N(4)-methyl-N(4)-phenylthiosemicarbazone (Hdptsc) (Fig. 1). The crystal structures of two complexes formulated as [Cu(dptsc)Br]2(2) and [Cu(dptsc)(l-N3)]2(3) are also described.

2. Experimental 2.1. Materials

Di-2-pyridyl ketone (Fluka) was used as received. Cu- Cl2Æ2H2O, CuBr2, Cu(OAc)2ÆH2O, Cu(NO3)2Æ2.5- H2O and NaN3 were commercial products of higher grade (Merck) and solvents were puriﬁed according to standard procedures. Elemental analyses were carried out using a Heraeus Elemental Analyzer and the molar conductance measurements of the complexes were carried out in DMF solvent at 28 ± 2C on a Century CC-601 digital conductivity meter with dip type cell and platinum electrode. Approximately 10³M solutions were used. The magnetic susceptibility measurements were made using a simple Gouy balance at room temperature using mercury tetrathiocyanatocobal- tate(II), Hg[Co(NCS)4] as calibrant. Infrared spectral measurements were done on a Shimadzu DR 8001 series FTIR instrument using KBr pellets for spectra in the region 4000–400 cm¹, and far IR spectra were recorded using polyethylene pellets in the 500–100 cm¹ region on a Nicolet Magna 550 FTIR instrument. An Ocean Optics SD 2000 Fiber Optic Spectrometer was used to measure solid-state reﬂectance spectra in the range 200–900 nm. ¹H NMR spectra were recorded using AMX 400 MHz FT-NMR Spectrometer with CDCl3

as solvent and TMS as the internal standard. EPR spectral measurements were carried out on a Varian E-112 X-band spectrometer using TCNE as standard.

2.2. Synthesis of ligand

The ligand, Hdptsc was synthesized as reported ear- lier [24]. A solution of di-2-pyridyl ketone (1.84 g,

10 mmol) in 5 ml methanol was treated with N(4)- methyl,N(4)-phenylthiosemicarbazide (1.81 g, 10 mmol) in 25 ml methanol and reﬂuxed for 2 h. On slow evaporation at room temperature, bright yellow crystals of Hdptsc separated out. These crystals were collected, washed with methanol and dried over P4O10in vacuo.

2.3. Synthesis of Cu(dptsc)Cl]2Æ3H2O (1)

Methanolic solutions of Hdptsc (0.347 g, 1 mmol) and CuCl2Æ2H2O (0.171 g, 1 mmol) were mixed and heated under reﬂux for 2 h. Dark blue-green crystals of complex 1 were obtained, which were separated, washed with methanol followed by ether and dried over P4O10in vacuo. Yield: 0.25 g (52%).

2.4. Synthesis of [Cu(dptsc)Br]2(2)

Hdptsc (0.347 g, 1 mmol) was dissolved in methanol (15 ml) and refluxed with a solution of CuBr2(0.223 g, 1 mmol) in 1:1 methanol–chloroform mixture. Blue crystals separated out, which were filtered, washed with methanol followed by ether and dried in vacuo. Colour- less single crystals suitable for X-ray diffraction were grown by slow evaporation of a 1:1:1 solution of methanol, chloroform and dichloromethane of the complex.

Yield: 0.175 g (33%).

2.5. Synthesis of [Cu(dptsc)(l-N3)]2(3)

Cu(OAc)2ÆH2O (0.199 g, 1 mmol) dissolved in methanol (15 ml) was added to Hdptsc (0.347 g, 1 mmol) and refluxed. NaN3 (0.0651 g, 1 mmol) was then added to the solution and further refluxed for 3 h. Dark blue crystals of3separated out, which were collected and dried in vacuo. Single crystals for X-ray diffraction were grown by slow evaporation of a 1:1:1 solution of methanol, chloroform and dichloromethane of the complex. Yield:

0.35 g (56%).

2.6. Synthesis of [Cu(dptsc)(NO3)]2ÆH2O (4)

A solution of Cu(NO3)2Æ2.5H2O (0.233 g, 1 mmol) in a methanol–chloroform mixture (20 ml, v/v) was added to Hdptsc (0.347 g, 1 mmol) and reﬂuxed for 2 h. The resulting crystals were washed with methanol, followed by ether and dried in vacuo. Yield: 0.35 g (65%).

2.7. X-ray crystallography

Single crystal X-ray diffraction measurements were carried out on a Bruker Smart Apex CCD diffractometer equipped with a fine-focused sealed tube. The unit cell parameters were determined and the data collections were performed using graphite-monochromated Mo Ka

N H₃C

HN

S

N

N N

Fig. 1. Structure of Hdptsc.

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(k= 0.71073 A˚ ) radiation. Both the crystals were found to be monoclinic with aP2₁/cspace group. Least square refinements of 17855 and 17083 reflections were done for compounds2and3, respectively. The data collected were reduced using theSAINTprogram[25]. The trial structures were obtained by direct methods [26] using SHELXS-86, which revealed the position of all non-hydrogen atoms and refined by full-matrix least squares onF²(SHELXL-97) [27]and the graphic tool was DIAMOND for windows [28]. All non-hydrogen atoms were refined anisotropi- cally, while the hydrogen atoms were treated with a mixture of independent and constrained refinements.

3. Results and discussion 3.1. Synthesis of complexes

Compounds1,2and4were prepared by direct reac- tion between the ligand and the corresponding metal

salts, while compound 3 was prepared by the displace- ment of the acetate in Cu(OAc)₂ÆH₂O by the azide ion. The complexes are mostly blue or blue-green col- oured, as expected with thiosemicarbazone coordination, resulting from the ligand to copper charge transfer bands [29]. Elemental analyses (C, H, N) data (Table 1) of the complexes are consistent with the 1:1:1 ratio of metal:thiosemicarbazone:gegenion and also reveals that compounds1and4are hydrated. Con- ductivity measurements in DMF solution (10³M) indicate that all the complexes are non-electrolytes suggesting anionic coordination to the metal centre.

3.2. Crystal structures of [Cu(dptsc)Br]2and [Cu(dptsc)(l-N3)]2

Structural studies of compound 2 reveal a three- dimensional copper-thiosemicarbazone network consisting of two units of [Cudptsc] with a distorted square pyramidal geometry around Cu(II), with the basal plane

Table 1

Elemental analyses, colours and molar conductivities of the Cu(II) complexes

Compound Colour Composition % (Found/Calc.) kMa leﬀb(BM)

Carbon Hydrogen Nitrogen

[Cu(dptsc)Cl]2Æ3H2O (1) Blue 48.54 (48.30) 3.65 (4.05) 14.71 (14.82) 32 1.63

[Cu(dptsc)Br]2(2) Blue 46.63 (46.58) 3.32 (3.29) 14.35 (14.30) 27 2.24

[Cu(dptsc)(l-N3)]2(3) Blue 50.53 (50.49) 3.62 (3.57) 24.82 (24.79) 27 1.98

[Cu(dptsc)(NO3)]2ÆH2O (4) Blue 46.67 (47.44) 3.45 (3.56) 17.98 (17.47) 24 2.56

a Molar conductivity, 10³M DMF at 298 K.

b Magnetic susceptibility.

Fig. 2. Molecular structure of complex2.

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consisting of one of the pyridyl nitrogens, azomethine nitrogen, thiolate sulfur and the bromine atom, while the pyridyl nitrogen of the second ligand unit occupies the apical position. A labelled representation of2is de- picted in Fig. 2 along with the structural data reﬁne- ments (Table 2) and selected bond distances and angles (Table 3). The bite angles N(1)–Cu(1)–N(3) (80.05), Br(1)–Cu(1)–N(1) (96.81), Br(1)–Cu(1)–S(1) (95.22) and S(1)–Cu(1)–N(3) (83.60) also support the distortion from square pyramidal geometry. The basal plane consisting of N(1), N(3), Br(1), S(1) and Cu(1) shows a maximum mean deviation of 0.3411 A˚ at N3. The Cu(II) atom is deviated at a distance of 0.3002 A˚ out of the mean plane towards the apical position. The thiosemicarbazone moiety comprising of N(5), C(12), S(1), N(4) and N(3) atoms and the pyridyl ring, Cg(3), make dihedral angles of 18.73 and 18.58, respectively, with the metal chelate CuN2BrS square plane. The loss of the proton bound to N(4) in Hdptsc produces a negative charge, which is delocalized on the thiosemicarbazone moiety. Delocalization of electron density over the entire thiosemicarbazone moiety is evidenced by the little marginal change in the C(6)–

N(3) and N(3)–N(4) bond distances in 2, compared to

that of the uncomplexed thiosemicarbazone [30]. The bond distance S(1)–C(12) is longer and C(12)–N(4) is shorter than the corresponding values in the free ligand, which support thiolate formation in the ligand on complexation [31].

The structural pattern of the discrete binuclear complex, [Cu(dptsc)(l-N3)]2(3)is best viewed as two end-on l1,1-azido bridged quasi-square pyramids. EachHdptsc unit acts as a tridentate chelate, coordinating through one of the pyridyl nitrogens, azomethine nitrogen and the thiolate sulfur to a single Cu(II) center which is in- volved in al1,1-azido bridging to an identical coordination network, thus materializing a penta-coordinated environment (Fig. 3). The base of each distorted square pyramidal unit is occupied by N(1), N(3), N(6) and S(1) atoms, with the apical position occupied by the N(6) atom of the second azido group. Each azido ligand func- tions as an electron donor to the coordination network through an end-on azido bridging mode. The centrosymmetric nature of the structure is revealed by an inversion center with the two azido-bridged angles, Cu–N–Cu (92.94) and N–Cu–N (87.06), being the same in both the units. The intramolecular Cu–Cu distance is found to be 3.303 A˚ and the axial Cu(1)–N(6)

Table 2

Crystal data, data collection and structure reﬁnement parameters for [Cu(dptsc)Br]2and [Cu(dptsc)(N3)]2

Parameters [Cu(dptsc)Br]2 [Cu(dptsc)(N3)]2

Empirical formula C19H16BrCuN5S C19H16CuN8S

Formula weight (M) 489.88 452.00

Temperature,T(K) 293(2) 293(2)

Wavelength (Mo Ka) (A˚ ) 0.71073 0.71073

Crystal system monoclinic monoclinic

Space group P21/c P21/c

Lattice constants

a(A˚ ) 9.029(4) 12.845(8)

b(A˚ ) 17.279(8) 12.512(7)

c(A˚ ) 13.217(6) 13.014(8)

a() 90.00 90

b() 97.228(8) 107.753(10)

c() 90.00 90.00

Volume,V(A˚³) 2045.6(16) 1992(2)

Z 4 4

Calculated density,q(Mg m³) 1.591 1.507

Absorption coeﬃcient,l(mm¹) 3.136 1.224

F(0 0 0) 980 924

Crystal size (mm) 0.41·0.13·0.11 0.30·0.12·0.09

hRange for data collection 2.36–27.98 1.66–28.04

Limiting indices 116h611,226k622,176l616 166h616,166k616,166l616

Reﬂections collected 17 855 17 083

Unique reﬂections 4877 [Rint= 0.0207] 4682 [Rint= 0.0265]

Completeness toh(%) 27.98 (99.2) 28.04 (97.1)

Maximum and minimum transmission 0.7242 and 0.3596 0.8978 and 0.7102

Reﬁnement method full-matrix least-squares onF² full-matrix least-squares onF²

Data/restraints/parameters 4877/0/296 4682/0/326

Goodness-of-ﬁt onF² 1.025 1.074

FinalRindices [I> 2r(I)] R1= 0.0431,wR2= 0.1202 R1= 0.0375,wR2= 0.0859

Rindices (all data) R1= 0.0558,wR2= 0.1296 R1= 0.0490,wR2= 0.0908

Largest diﬀerence peak and hole (e A˚³) 1.455 and0.538 0.418 and0.218

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distance is found to be 2.5628 A˚ . The nitrogen atoms N(3) and N(6) are closer to the Cu(II) center with bond distances Cu(1)–N(3) (1.964 A˚ ) and Cu(1)–N(6) (1.956 A˚ ) compared to Cu(1)–N(1) (2.027 A˚) and Cu(1)–S(1) (2.2603 A˚ ). A maximum deviation of 0.1649 A˚ at N(1) is revealed at the base of the square pyramid with the central Cu(II) atom positioned 0.0689 A˚ above the mean plane. The pyridyl ring Cg(1) is less deviated from the basal plane (the dihedral angle between the two least square planes is 3.00), when compared to the thiosemicarbazone moiety, which devi- ates more, at a dihedral angle of 13.51 from the basal CuN3S plane. The dimer is located on an inversion centre and the bridging Cu2N2network is perfectly planar.

Similar to2 above, the C(6)–N(3) and N(3)–N(4) bond

distances in3reveal extensive delocalization over the entire binuclear coordination framework.

The diverse p–p stacking, C–H- - -p and ring–metal interactions give rise to polymeric chains in the unit cells of2and3(Figs. 4 and 5). The shortestp–pinteractions are perceived at 3.3631 A˚ for Cg(2)–Cg(2)ⁱ [i = 1x, 1y, 1z] in 2. Ring–metal interactions Cg(2)- - - Cu(1)ⁱⁱⁱ [iii =x, 1/2y, 1/2 +z] are observed in 3 at a distance of 3.774 A˚ . Two C–H- - -p interactions each are shown in the unit cells of 2 and3. They are C(8)–

H(8)- - -Cg(1)^iv [dH- - -Cg= 3.0853 A˚ ; iv =1 +x,y,z]

and C(10)–H(10)- - -Cg(5)^v [dH- - -Cg= 2.6471 A˚ ; v = 1 +x, 1/2y,1/2 +z] for 2 and C(10)–H(10)- - - Cg(3)^vi [dH- - -Cg= 2.9888 A˚ ; vi = 1x,y, 1z] and C(15)–H(15)- - -Cg(2)^vii [dH- - -Cg= 2.9537 A˚ ; v = 1x, 1/2 +y, 1/2z] for compound3.

3.3. Spectrochemical studies

The ma(N–H) vibrations of the imino group are observed at ca. 3428 cm¹in the IR spectrum of Hdptsc.

In the ¹H NMR spectrum, the N(4)H resonance, at a downﬁeld value of 14.68 ppm, is consistent with the intramolecular N(4)–H- - -N(2) hydrogen bonding revealed by its crystal structure [30]. A strong band at 1580 cm¹ in the IR spectrum of Hdptsc corresponds tom(N3–C6) stretching, which suﬀers a positive shift on complexation due to a change in bond order [31,32]

(Table 4). The absence of a m(S–H) band around 2565 cm¹ supports the predominant thione form of the ligand in the solid state, while the lack of an (N–H) stretching in the spectra of the complexes endorses the ligand coordination to Cu(II) ion in the deprotonated thiolate form. A medium band around 644 cm¹ indicates out-of-plane pyridyl ring vibrations q(py) of Hdptsc, which suﬀers a positive shift in all complexes.

The IR bands observed at 1360 and 793 cm¹in the ligand have signiﬁcant contributions from C@S stretching and bending vibrations, and are shifted to lower wave- numbers on complexation [33]. Solid state reﬂectance spectrum of the ligand shows an absorption maximum in the region 37000 cm¹ attributed to intra-ligand p^* ptransitions of the pyridyl ring and imine function of the thiosemicarbazone moiety (Table 5). The peaks at ca. 27 000 and 30 030 cm¹indicatep^* n transitions of the thioamide function, which are shifted to higher energy values upon complexation. As expected, the Cu(II) S charge transfer transitions[34]are observed at ca. 23 000 cm¹. For all the compounds, the peaks observed at ca. 17 000 and 14 000 cm¹are assigned to dxz, dyz d_x²_y² and d²_z d_x²_y² transitions in an elongated square based pyramidal geometry[35,36]. Magnetic moment values for the bromo (2) and nitrato (4) complexes are greater than the spin only value for a dimeric system, since a larger Cu–Cu distance, revealed by the crystal structure in2, render the electron spins parallel resulting

Table 3

Comparison of selected bond lengths (A˚ ) and bond angles () of Hdptsc, [Cu(dptsc)Br]2and [Cu(dptsc)(N3)]2

Hdptsc [Cu(dptsc)Br]2 [Cu(dptsc)(N3)]2

S(1)–C(12) 1.668 1.721 1.744

N(3)–C(6) 1.295 1.298 1.299

N(3)–N(4) 1.361 1.354 1.370

N(4)–C(12) 1.377 1.343 1.326

N(5)–C(12) 1.350 1.344 1.354

Cu(1)–S(1) 2.2565 2.2603

Cu(1)–N(1) 2.019 2.027

Cu(1)–N(3) 1.979 1.964

Cu(1)–N(2) 2.352

Cu(1)–Br(1) 2.4134

Cu(1)–N(6) 1.9561

Cu(1)–N(6)a 2.5628

C(6)–N(3)–N(4) 120.87 118.9 120.36

N(3)–N(4)–C(12) 118.72 111.1 111.68

N(5)–C(12)–N(4) 113.74 115.6 115.02

N(5)–C(12)–S(1) 123.48 118.9 119.09

N(4)–C(12)–S(1) 122.77 125.4 124.89

N(1)–Cu(1)–N(3) 80.05 80.56

S(1)–Cu(1)–N(1) 163.28 160.88

S(1)–Cu(1)–N(3) 83.60 84.95

S(1)–Cu(1)–N(2) 100.88

N(2)–Cu(1)–N(1) 89.00

N(2)–Cu(1)–N(3) 113.93

Br(1)–Cu(1)–S(1) 95.22

Br(1)–Cu(1)–N(1) 96.81

Br(1)–Cu(1)–N(2) 97.60

Br(1)–Cu(1)–N(3) 148.15

S(1)–Cu(1)–N(6) 100.81

S(1)–Cu(1)–N(7) 81.74

S(1)–Cu(1)–N(6)a 104.53

N(1)–Cu(1)–N(6) 94.28

N(1)–Cu(1)–N(7) 112.12

N(1)–Cu(1)–N(6)a 87.77

N(3)–Cu(1)–N(6) 173.80

N(3)–Cu(1)–N(7) 166.63

N(3)–Cu(1)–N(6)a 89.29

N(6)–Cu(1)–N(7) 19.40

N(6)–Cu(1)–N(6)a 87.06

N(7)–Cu(1)–N(6)a 95.25

Cu(1)–N(6)–Cu(1)a 92.94

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in a high magnetic moment. The azido (3) and the chloro complex (1) show magnetic moment values close to the spin only value per copper for a dimer, suggesting the ab-

sence of spin–orbit coupling and anti-ferromagnetic interactions. A medium band at 245 cm¹ in the IR spectrum of2 is indicative of the coordinated bromine.

Fig. 3. Molecular structure of complex3.

Fig. 4. Molecular packing diagram of2viewed down thecaxis.

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Compound 3 exhibits strong bands at 2041 and 1310 cm¹corresponding to themaandmsof the coordinating azido group [37]. The prominent bands at 699 cm¹ and a weak band at 444 cm¹, assigned to d(NNN) and m(Cu–N), suggest a non-linear Cu–N–N–

N bond[34]. Bands corresponding tom1,m2,m4at 1010, 1284 and 1385 cm¹, respectively, are observed in 4

whereas a m1+m4 combination band at ca. 1750 cm¹ indicates monocoordinated nitrato group[37]. Magnetic moment measurement of4supports its potential dimeric disposition where the magnetic susceptibility value is found to be higher than that of a coupled Cu(II) ion pair, which is presumed with the association of small amounts of paramagnetic impurities in the compounds[38].

Fig. 5. Molecular packing diagram of3viewed down thebaxis.

Table 4

Selected IR bands (cm¹) with tentative assignments for the complexes

Compound m(C@N) +m(N@C) m(N–N) m(C@S) d(C@S) d(o.p) m(Cu–N) m(Cu–S) m(Cu–N) py m(Cu–X^#)

Hdptsc 1580 s 1051 w 1360 s 793 m 644 m

[Cu(dptsc)Cl]2Æ3H2O (1) 1592 m 1004 w 1305 m 781 w 683 w 419 s 325 s 353 m 303 m

[Cu(dptsc)Br]2(2) 1590 w 1002 w 1306 m 781 m 692 m 419 s 328 m 350 m 245 m

[Cu(dptsc)(l-N3)]2(3) 1594 m 1011m 1310 sh 778 w 699 m 444 s 332 sh 351 m 315 m

[Cu(dptsc)(NO3)]2ÆH2O (4) 1592 w 1005 m 1308 m 775 m 692 m 416 s 325 m 355 m 444 w

Table 5

Electronic spectral data (cm¹) of Hdptsc and the complexes

Compound d–d C T p* n p* p

Hdptsc 26882 s,

30030 sh 37543 s,

32787 sh

[Cu(dptsc)Cl]2Æ3H2O (1) 17636 sh, 24814 sh, 32154 s 37313 s

14084 sh 23201 s 30395 b

[Cu(dptsc)Br]2(2) 17699 sh, 28169 sh

14556 sh 23310 sh, 32467 s, 37453 s

22831 s,b 31648 sh

[Cu(dptsc)(l-N3)]2(3) 17211 sh, 26882 sh, 32573 sh,

144450 sh 22676 s 30864 sh

[Cu(dptsc)(NO3)]2ÆH2O (4) 17007 sh, 23095 s,b,

14104 sh 16949 sh 30487 s,b 38314 sh

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3.4. Electron paramagnetic resonance spectra

EPR spectra of the samples with 100-kHz field mod- ulation at room temperature were recorded in the X-band. An isotropic spectrum with a broad signal is obtained for 3 and the remaining complexes gave axial spectra in the solid state at room temperature. The copper(II) ion, with a d⁹configuration, has an effective spin ofS= 1/2 and is associated with a spin angular momen- tumms= ±1/2, leading to a doubly degenerate spin state in the absence of a magnetic field. In a magnetic field the degeneracy is lifted between these states and the energy difference between them is given byE=hm=gbH, where his Plancks constant,mis the frequency,gis the Landes splitting factor (equal to 2.0023 for a free electron),bis the Bohr magneton andHis the magnetic field. For the case of a 3d⁹copper(II) ion, the appropriate spin Ham- iltonian assuming aB1gground state[36] is given by:

H^ ¼b½g_kHzSzþg_?ðHxSxþHySyÞ þAkIzSzþA?ðIxSxþIySyÞ

The EPR spectra of the compounds1,2and4in the polycrystalline state at 298 K show typical axial spectra with slightly diﬀerentgiandg^features. The variations ingvalues indicate that the geometry of the compound is aﬀected by the nature of the coordinating gegenions (Table 6). The spectrum of compound3in the polycrystalline state (298 K) shows a broad signal indicating only one g value, 2.0638, due to dipolar broadening and enhanced spin lattice relaxation.

The geometric parameter G calculated as G= (gi2.003)/(g_^2.003) is a measure of the exchange interaction between copper centers in the polycrystalline compound. It is perceived that, if G> 4, the exchange interaction is negligible and vice versa in the complexes.

All complexes withgi>g^> 2.003 andGvalues falling within the range of 2–4 are consistent with a d_x²_y² ground state in a square planar or square pyramidal geometry. Thegvalues are found to be lower than those reported for copper(II) complexes prepared from a substituted 2-benzoylpyridine thiosemicarbazone [39], indicating a stronger metal-ligand bonding. Absence of half ﬁeld signals for the compounds reinforces the assumption of very weak super exchange interactions.

The solution spectra of complexes 1 and 4 were recorded in DMF at 298 K. They are isotropic in nature with well-resolved four hyperfine lines. The hyperfine splitting is due to the interaction of the electron spin with the copper nuclear spin (⁶⁵Cu, I= 3/2). There are indications of nitrogen superhyperfine splitting in the high field component in some spectra. TheAisoandgiso

values show variations in1and4, their values indicating dissimilarity in bonding in the complexes.

The spectra of complexes1,3and4in DMF at 77 K show rhombic features with threegvaluesg1,g2, andg3

where g3>g2>g1. It is observed that the g values for complexes in the solid state at 298 K and in DMF at 77 K are not much diﬀerent from each other, hence the geometry around the copper(II) ion is unaﬀected on cooling the solution to liquid nitrogen temperature.

In the low ﬁeld region, the chloro and nitrato complexes show six hyperﬁne lines that are moderately resolved and perpendicular features overlapping with seventh one suggesting a dimeric state with two copper centers.

Theg3values of the complexes are less than 2.3 and they assign considerable covalent character to the M–L bonds [40]. For the compounds 1, 3 and 4, the lowest g value (g₁) is less than 2.04 indicating a compressed rhombic symmetry with all axes aligned parallel and is consistent with distorted trigonal bipyramidal stereo- chemistry or a compressed axial symmetry or rhombic symmetry with slight misalignment of the axes. In the spectra with g3>g2>g1, rhombic spectral values R= (g2g1)/(g3g2) may be signiﬁcant. If R> 1, a predominant d_z² ground state is present. IfR< 1, a predominant d_x²_y²ground state is present and whenR= 1, then the ground state is approximately an equal mixture of d_z² and d_x²_y², the structure is intermediate between square planar and trigonal bipyramidal geometries. All the present complexes have values R< 1 suggesting a distorted square based pyramidal geometry with a dx²y² ground state. These observations are consistent with g values of the corresponding complexes in the polycrystalline state at 298 K and further supports the distorted square pyramidal geometry around copper(II) ion in these complexes.

Spin Hamiltonian parameters are calculated from the frozen state EPR and electronic spectra (Table 7). The

Table 6

EPR spectral assignments (experimental) for the copper(II) complexes

Compound Solid (298 K) DMF(298 K) DMF (77 K)

gi giso g_^ giso Aiso AN g3/gi g2 g1/g_^ gav Ai^a A_^(N)^a [Cu(dptsc)Cl]2Æ3H2O (1) 2.1422 2.0576 2.0446 43.33 15 2.2224 2.0597 1.9808 2.0876 63.33 11.66

[Cu(dptsc)Br]2(2) 2.1349 2.0602 2.1423 2.0612 2.0882

[Cu(dptsc)(l-N3)]2(3) 2.0638 2.1961 2.0631 1.9868 2.0820 175 22

[Cu(dptsc)(NO3)]2ÆH2O (4) 2.1484 2.0384 2.1047 48.33 20 2.1816 2.0341 1.9723 2.0627

a A values in 10⁴cm¹.

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orbital reduction factors were estimated using the following expressions[41].

K²_k¼ ðg_k2.0023ÞDEðdxydx²y²Þ=8k0;

K²_?¼ ðg_?2.0023ÞDEðdxz;yzd_x²_y²Þ=2k0;

wherek0is the spin–orbit coupling constant and has a value of828 cm¹for Cu(II) d⁹systems.

According to Hathaway[42],Ki=K_^= 0.77 for pure rbonding andKi<K_^for in-plane bonding, while for out-of-plane bonding Ki>K_^. It is seen that for com- plexes1and4Ki>K_^, indicating stronger out-of-plane pbonding.

4. Supplementary data

Crystallographic data for structural analysis has been deposited with the Cambridge Crystallographic Data center, CCDC 231110 for compound [Cu2(dptsc)2Br2] (2) and CCDC 231109 for compound [Cu2(dptsc)2(N3)2] (3). Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 IEZ, UK (fax: +44-1223-336-033;

e-mail: [email protected] or http://www.ccdc.

cam.ac.uk).

Acknowledgements

The authors are thankful to the Regional Sophisti- cated Instrumentation Center, CDRI, Lucknow, India, for the elemental analysis and Sophisticated Instruments Facility, IISc Bangalore for NMR measurements.

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Table 7

Spin Hamiltonian and orbital reduction parameters of the complexes1 and4

Compound [Cu(dptsc)Cl]2Æ3H2O (1) [Cu(dptsc)(NO3)]2ÆH2O (4)

gi/gzz(g3) 2.2224 2.1816

gyy(g2) 2.0597 2.0341

gxx(g1) 1.9808 1.9723

gav (77k) 2.0876 2.0627

gav(solid) 2.0858 2.0750

R^a 0.404 0.98

Ki 0.6686 0.6357

K^ 0.6534 0.6030

a R= (g2g1)/(g3g2).

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